Haeffner discovered that the isotopes of liquid mercury could be separ
ated by an electric field, with the lighter isotopes being preferentia
lly enriched near the anode. We consider the analogous problem for a g
aseous, fully-ionized, binary isotopic plasma and it is demonstrated t
hat isotopic separation occurs in the same way, i.e. the lighter isoto
pes are preferentially located near the anode. A previous analysis by
Frie is discussed.